Improved low-temperature activity for marine selective catalytic reduction systems

Abstract

Through the international work carried out recently by International Maritime Organization Marine Environment Protection Committee, the need for improved selective catalytic reduction systems for marine applications at low and transient temperatures has been acknowledged. It is of importance to improve the low-temperature activity to achieve compliance with the upcoming stricter International Maritime Organization NOX regulation (Tier III, 80% reduction) for ships. This study therefore investigates the possibilities to improve the low-temperature activity for a commercial vanadium-based urea-selective catalytic reduction catalyst for marine applications. By altering the NO2/NOX ratio, the reaction proceeds in the fast selective catalytic reduction reaction regime, which is especially beneficial at low and transient temperatures and in combination with SO2 and H2O. For both steady-state and transient modes, it is suggested that an improved low-temperature activity for the marine selective catalytic reduction system can be achieved by increasing the NO2/NOX ratio to 50%, whereby utilizing the fast selective catalytic reduction reaction scheme. The fast selective catalytic reduction can significantly improve the NOX reduction already at 250 °C in the presence of water or sulfur. The fast selective catalytic reduction will, however, require an increased NO2 fraction in the exhaust gases, preferably 50%, which may be achieved by placing an oxidation catalyst upstream of the selective catalytic reduction. The use of an oxidation catalyst upstream of the selective catalytic reduction can offer a possibility to achieve International Maritime Organization Tier III compliance at low and transient temperatures, but to avoid formation of ammonium nitrates and possible ammonium sulfates, temperatures should be kept above 250 °C in combination with marine fuels with a maximum of 0.10 wt% S.